Chronic myeloid leukemia (CML) is a myeloproliferative disorder characterized by the presence of Bcr-Abl oncogenic reciprocal translocation t(9,22)(q34:q11) (Bartram, et al.; Nature; (306); 277-280; 1983). This translocation is present in 90-95% of CML patients and leads to the expression of the fusion protein Bcr-Abl (P-210 kDa) with constitutive protein-tyrosine kinase activity (Shtivelman, et al.; Nature; (315); 550-554; 1985, Groffen, et al.; Cell; (36); 93-99; 1984). Bcr-Abl is essential for malignant transformation and triggers several cellular signaling pathways (e.g. CrkL, STAT5, PI3K/AKT) to regulate cell proliferation, differentiation, migration, survival and DNA repair (Lugo, et al.; Science; (247); 1079-1082; 1990). Targeting Bcr-Abl has become an important strategy for CML treatment (An, et al.; Leuk Res; (34); 1255-1268; 2010). Imatinib (STI571, Gleevec, Norvartis) effectively inhibits tyrosine kinase activity by occupying the adenosine triphosphate (ATP)-binding pocket of Bcr-Abl, thus abrogating subsequent signal transduction and is the preferred first-line therapy for CML (An, et al.; Leuk Res; (34); 1255-1268; 2010).
Although, treatment for CML has seen tremendous advance following the discovery of imatinib and other BCR-ABL tyrosine kinase inhibitors (TKI), however, complete molecular response, amounting to undetectable BCR-ABL transcript is not achieved in majority of the CML patients (Prost, et al.; Nature; (525); 380-383; 2015). TKI-resistance can occur due to mutations in BCR-ABL, however, in approximately 50% cases BCR-ABL-independent mechanisms including TKI-refractory leukemia stem cells (LSC) contribute to resistance, recurrence and disease progression (Prost, et al.; Nature; (525); 380-383; 2015). Recently, anti-diabetic thiazolidinedione peroxisome proliferator-activated receptor gamma (PPARγ) agonists; pioglitazone in particular, was reported to erode quiescent LSCs by targeting signal transducer and activator of transcription 5 (stat5) expression in preclinical and clinical settings (Prost, et al.; Nature; (525); 380-383; 2015, Glodkowska-Mrowka, et al.; Blood Cancer J; (6); e377; 2016). Unfortunately, recently found association of pioglitazone with bladder cancer (Tuccori, et al.; BMJ; (352); i1541; 2016) led to its withdrawal in France and Germany. Rosiglitazone, on the contrary did not increase bladder cancer incidence, but is associated with severe cardiovascular risks (Nissen, et al.; N Engl J Med; (356); 2457-2471; 2007).
Clofazimine (CFZ) is a riminophenazine leprosy drug which is also effective against multidrug-resistant and extremely drug-resistant tuberculosis (Gopal, et al.; Int J Tuberc Lung Dis; (17); 1001-1007; 2013). CFZ imparts its anti-bacterial actions by generation of reactive oxygen species (ROS), particularly superoxides and H2O2(Cholo, et al.; J Antimicrob Chemother; (67); 290-298; 2012). CFZ also displays anti-inflammatory properties that is important for its antileprosy effects including suppression of erythema nodosum leprosum and leprosy-associated immune reactions (Cholo, Steel, Fourie, Germishuizen and Anderson; J Antimicrob Chemother; (67); 290-298; 2012). Clinical studies have also found CFZ to be effective against various autoimmune diseases including discoid lupus erythematosus, Crohn's disease, ulcerative colitis, psoriasis, Meischer's granuloma and graft-versus-host disease (Ren, et al.; PLoS One; (3); e4009; 2008). CFZ is reported to impart its immonomodulatory activities by blocking KV1.3 voltage gated potassium channel (Ren, et al.; PLoS One; (3); e4009; 2008) and thereby inhibit chronic lympocytic leukemia cells (Leanza, et al.; EMBO Mol Med; (4); 577-593; 2012, Leanza, et al.; Leukemia; (27); 1782-1785; 2013) and based on its KV1.3 modulatory properties CFZ and its derivatives have been patented for various autoimmune diseases (Liu et al, U.S. Pat. No. 8,669,257 B2). CFZ has also been evaluated in chronic myeloid leukemia patients and was found to reduce granulocytes, basophilic leucocytes and histamine in peripheral blood of these patients (Brandt; Scand J Haematol; (9); 159-166; 1972). However, this report does not elucidate if the beneficial effects of CFZ was routed through direct apoptosis/differentiation-inducing effects of CFZ on CML cells or simply an indirect outcome of its immunomodulatory activities. Further, CFZ has not been tested in BCR-ABL TKI-inhibitor-resistant CML cells and its effect in LSCs or quiescent LSCs alone or in combination with BCR-ABL TKI inhibitors is not known. Furthermore, no report is available in public domain where the ability of CFZ to alter the bioavailability of TKI inhibitors has been investigated.